1. Field of the invention
This invention relates in general to flexible pipes for oil and gas well installations and in particular to those used for subsea oil and gas production.
2. Description of the Prior Art
In well production operations, flexible pipes are used for various purposes such as flowlines. A flexible pipe is herein considered to be a pipe having, at least during transportation and installation, sufficient flexibility along its longitudinal axis to accept a minimum radius of curvature around 6.5 times the diameter of the pipe. These flexible pipes must be able to withstand high internal pressures, as much as 5,000 psi. In underwater use for subsea well production, they must be able to withstand crushing due to external pressures as well as tensile loading during installation. In some installations, high temperatures are also encountered. In addition flexing should not affect the strength properties of the pipe.
Available flexible pipes rely on an inner polymeric tube to contain the high pressure fluid. While this works well enough at low temperature, high temperatures combined with pressure may make the polymers permeable to gas. This situation results in varying degrees of loss of transported fluid which accumulates between the inner and outer sheath which could cause the collapse of the inner sheath in case of rapid depressurization. Also this could cause accelerated corrosion of the reinforcing structure in case of hydrocarbon fluid containing corrosive agents such as H2S, CO2, etc. . . . In addition polymeric tubes are generally limited to temperatures less than 100 to 130 degrees C. depending of the nature of the polymer.
To withstand high temperature service, a metallic internal liner would be highly desirable. However, it has not been used because of the inherent rigidity of metal incompatible with the requirement of flexibility of a flexible pipe. Some attempts have been made to use a metallic liner with a corrugated wall. Since in these attempts, it was only considered to operate within the yield strength of the material the wall thickness had to be very thin leading to a wave height to wall thickness ratio of the corrugations exceeding 8. This high ratio causes very deep corrugations, which take up a large radial portion of the flexible pipe wall thickness. Also the resulting thin wall thickness limits the pressure capability and offers reduced service life in case of corrosive fluids.
Flexible pipes also have a reinforcing structure surrounding the fluid containing tube. Typical reinforcing structure includes a combination of at least two cross helical tensile armors wrapped around a helical winding of interlocking strips. While workable, this structure has many parts and is expensive to manufacture.